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The detection and quantitation of pesticides by enzyme linked immunosorbent assay (ELISA) was compared with classical analytical methods. ELISA was used to evaluate the body burdens of alachlor (15972608) or its metabolic products in urine collected from 20 pesticide applicators, seven haulers/mixers, and eight employees with limited exposure to pesticides. Alachlor was too small to be immunogenic itself, and was detected as an alachlor-mercapturate. ELISA and high pressure liquid chromatography gave statistically significantly different results and showed a positive bias by ELISA. The bias probably related to similarities in structure between human alachlor metabolic products excreted in urine and primary immunogen used to produce the polyclonal antibodies. In detecting circulating antibodies, the half life of the antibody could be longer than the half life for elimination of the parent compound. This gave a legacy or history of exposure rather than direct biomonitoring, which was subject to false negatives, but not false positives. Legacy biomonitoring was useful for evaluating engineering controls for reducing exposures. Within limitations, antibody levels were related to exposure levels. The authors conclude that legacy biomarkers may reflect exposure in the absence of detectable chemicals in urine or blood.